![]() ![]() Siemens data blocks however are retentive unless defined not to be. Omron separates its retentive bits into “holding” relays and non-retentive “CIO”, and its data into the retentive DM Area and non-retentive “Work Area” Siemens allows its general “marker” memory to be assigned as retentive or non-retentive and defaults to only 16 bytes of retentive marker memory. Other PLC platforms assign some parts of RAM to be “retentive” and other parts non-retentive. This means that the values in data registers will be retained and the program will start in its last state. Some platforms, however, ensure that the data is kept intact even when power is lost by use of battery backed RAM. Not all PLC platforms back up the user memory with a battery or other energy storage device, data memory may be lost when a processor loses power. When the PLC is powered on, the program is loaded from non-volatile RAM cards into the user memory of the controller. Though the program can be saved on Flash or SD RAM cards without a battery, the data exchange rate is too slow to use this for the actual interfacing of the program with its data. In the previous hardware section it was explained that there are volatile and non-volatile areas of memory, and that the volatile part of memory needs a battery, “super capacitor” or other rechargeable energy storage module to hold its program and/or data. PLC Memory consists of the operating system and firmware of the processor and modules, and the program and data that is used by the programmer. On these older battery-backed platforms, if the battery died, the program was lost. ![]() Older PLC systems required a battery or a “super-capacitor” to back up the program when power was removed for long periods of time, newer platforms can save the program to non-volatile memory such as CompactFlash and Secure Digital (SD) RAM. ![]() The program itself is in one area of RAM and must be kept in memory even when the PLC is powered off. This type of memory may be volatile or non-volatile, and it can (and will) be overwritten often. Program and data memory in a PLC is contained in “RAM” (Random Access Memory). As this is an evolutionary process, it will undergo many changes. Todays post is an excerpt from part of the course, and I welcome any comments or critiques. I have nearly doubled the size of the material since then including a fairly extensive computer and PLC history, which I may post excerpts from later. I was able to test a prototype a few weeks ago with a regular customer of mine down in Miami, and it went pretty well. As I have mentioned previously, I have been working on a generic PLC course that I hope to release in some kind of online software format soon. ![]()
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